I am running Vista Home Premium 64 with a butt load of disc space left, and the other 4 episodes work just fine. I tried the DX updater but I already have the latest DirectX version so it won't update.

I also ran a disc cleanup program that came with my McAfee security center which cleaned up some registry files, etc.

I have deleted and re-downloaded the program several times, and tried running the setup as Administrator, all to no avail.